1. Field of the Invention
The present invention relates in general to a control apparatus for controlling a vehicle drive apparatus which is equipped with a differential mechanism serving as a speed variable transmission owing to its differential action. More particularly, the invention is concerned with techniques for improving an efficiency of electricity generation while a drive power transmitting path is being disconnected.
2. Discussion of Related Art
There is known a hybrid vehicle drive apparatus including: (i) a power transmitting member; and (ii) a differential mechanism having a first element connected to an internal combustion engine, a second element connected to a first electric motor having an electricity generating function, and a third element connected to the power transmitting member and a second electric motor which is connectable to the first electric motor through an electric path, as disclosed in JP-2000-238555A, JP-2003-130203A and JP-2003-127681A. The differential mechanism of the drive apparatus serves as a power distributing mechanism for distributing a drive power or force of the engine to the first electric motor and the power transmitting member. The differential mechanism is constituted by a planetary gear set whose differential action permits a major portion of the drive force of the engine to be mechanically transmitted to drive wheels and also permits the rest of the drive force to be electrically transmitted from the first electric motor to the second electric motor through the electric path, so as to serve as a speed variable transmission capable of electrically changing its speed ratio, thereby making it possible to drive the vehicle with the engine kept in an optimum operating state with a reduced fuel consumption. Further, as other prior arts relating to the present invention, JP-H09-98516A discloses a technique for reducing a fuel consumption of a hybrid vehicle, and JP-2001-41073A discloses a technique for improving an efficiency of an electricity generation performed by a generator which is driven by a vehicle engine. Specifically, JP-2001-41073A teaches releasing a clutch during the electricity generation, for avoiding of transmission of a rotary motion of the engine to a fluid coupling (torque converter) which is disposed on one of opposite sides of the clutch remote from the engine, so as to improve the electricity generation efficiency.
In the above-described hybrid vehicle drive apparatus, when the electricity generation is intended with the drive apparatus being placed in its non-driving position such as a neutral position, the rotational speed of the second element has to be increased to such a value that permits the rotational speed of the power transmitting member to be zeroed. This means that the rotational speed of the first electric motor connected to the second element can not be freely determined, namely, the first electric motor as a generator can not be operated at such an operating point that maximizes the efficiency of the electricity generation. That is, in the above-described hybrid vehicle drive apparatus, there is a limit to improvement of the generation efficiency. In view of this, there has been a demand for a control apparatus capable of maximizing the efficiency of the electricity generation as intended while the drive apparatus is placed in the non-driving position, in the vehicle drive apparatus equipped with the differential mechanism serving as the speed variable transmission owing to its differential action.